The separation of molecules in biological fluids is a basic procedure which is involved in nearly all biotechnology and pharmaceutical industry processes, both in laboratory scale and in production scale. It is also important in clinical diagnosis and disease treatment, for example in hemodialysis, and the most elemental device for solute separation is a porous membrane filter. Thus, a revolutionary advance in filter technology has the potential to impact many areas of industry and human health.
Filters that are able to catch nanosize objects, such as viruses and bacteria, are also of great importance in air and water purification systems and low temperature sterilization of drug solutions. Such air filtering is used for example in airplanes and in military devices in order to protect from biological and chemical weapons of mass destruction used in war or terrorism attacks. In addition to air, efficient filtering of water and liquids from nanosized contamination is of equal importance. First of all, this would increase drinkable water sources in developing countries, but there are also Research and Development (R&D) efforts directed towards filtering off viruses such as HIV out of other liquids, including human blood serum.
Typical filter materials are made as woven matrices of plastic or cellulose polymers. Filters manufactured in this manner naturally contain a wide distribution of pore sizes and the smallest pores will eventually clog with small molecules of the filtrate. The abundance of small pores and large filter thicknesses are the two major sources of resistance to flow across membrane filters as described in Tong et al., “Silicon Nitride Nanosieve Membrane,” Nano Letters 4:283-287 (2004). Moreover, such filters have very limited resistance to thermal conditions (they usually degrade at temperatures above 200° C.) or chemical environments (they are affected by solvents, acids and bases).
Recent advances in nanotechnology allow fabrication of very thin filter membranes with high density and high precision of pore sizes down to a few nanometers in diameter. However, such fabrication involves very expensive equipment (such as electron beam lithography (EBL) or focused ion beam drilling (FIB)) and is very inefficient when a large number of nanopores needs to be produced, because they are based on serial process (i.e. one nanopore has to be produced after another).
U.S. patent application Ser. No. 11/414,991 discloses a process for the manufacturing of a porous nanoscale membrane. In the final step a plurality of spaced nanopores are formed by removal of oxide masks. However the produced nanopores, as disclosed in the patent, have relatively low density and broad size distribution of the nanopores, especially in case of larger pores (>20 nm).
U.S. Pat. No. 5,753,014 discloses membrane filters comprising a membrane provided with pores having a pore size between 5 nm and 50 □m, methods for manufacturing such membrane filters and uses thereof. The membrane layer is provided with pores by means of a patterned auxiliary layer which is brought in the desired pattern by a photolithographic or imprinting technique. The pattern is transferred to the membrane layer by etching. It is mentioned that one way of producing the membrane pores is lifting off parts of the membrane layer and subjacent parts of the patterned masking layer. This lift-off process increases the surface roughness of the membrane thereby rendering it less suitable for certain applications, e.g. medical or bio-medical applications.